Vortex flowmeters are commercially available in which the Karmen vortex principle is used to detect the vortex shedding frequency from which the fluid flow rate may be determined. In such apparatus, it is known that a non-streamlined or bluff body placed in the flowstream will shed vortices. The passage of these vortices along a point in the flowstream, corresponding to the vortex shedding frequency, can be detected and used as a measure of the rate of fluid flow. Thus, as a vortex forms and is shed from the bluff body, a corresponding fluid pressure gradient will exist near the bluff body which can be detected by suitable detectors.
Currently available vortex flowmeters use various apparatus to detect the minute Karmen vortex pressure gradients. Some such existing sensing structure utilize strain gauges or piezoelectric sensors which are directly on the bluff body or otherwise immersed in the fluid flow path and are therefore undesirably subjected to possibly high temperature fluids. In other devices, the sensor is mounted outside of the flow path and is acted on by the bluff body or other force sensing/transmitting member passing through the fluid pipeline wall to contact the sensor. However, this configuration is disadvantageous in that hydrostatic noise from the flowing fluid is coupled through the pipeline wall to the sensors making it more difficult to detect the very minute vortex pressure gradients from the hydrostatic noise.
On the other hand, it is advantageous to have a large force collecting area for sensors desirably located outside of the pipeline, as this enhances the ability to detect the low level vortex pressure gradients. Thus, a vortex flowmeter designer placing the sensors outside of the pipeline is faced with making a design compromise between having a large force collecting area to increase the ability to detect low level pressure gradients and the desire to have a very small force collecting area to reduce the undesired transmission of hydrostatic noise from the fluid in the pipeline to the sensors outside of the pipeline.
Another disadvantage of currently available or suggested vortex flowmeters is that the force sensing member which intrudes into the flowstream must normally be very solid in order to withstand the fluid flow velocity, and yet this same structure must be very sensitive to the small force inputs due to the minute vortex created pressure gradients. Again, design compromises must be made by those skilled in the art so that such prior force sensing members are design compromises being less than desired for sensing vortex pressure gradients in the fluid and also being less than desired in transmitting such sensed pressure gradients to the sensors outside of the pipeline.
Accordingly, it is desired to provide a vortex force transmitter for use in a vortex fluid flowmeter wherein the force sensor is located outside of the pipeline; a large force collecting area can be used while reducing the sensing of hydrostatic noise; and no design compromises of the force sensing/transmitting member are required.